This invention is directed to a monopropellant thruster of the type where a liquid propellant is decomposed into a gas mixture with a net increase in temperature which is expelled to produce thrust. Hydrazine is a particular liquid.
Decomposition of liquid hydrazine is known in the prior art. Liquid hydrazine is discharged into contact with a catalyst material which initiates and sustains a net exothermic decomposition into gaseous products. The hydrazine liquid decomposes exothermically into gaseous ammonia, nitrogen and hydrogen. The ammonia decomposes endothermically into nitrogen and hydrogen, but the overall reaction gives off heat. In the present state of the art, the best dissociation catalyst for hydrazine is manufactured by the Shell Oil Company and is commercially available under the name "Shell 405 Catalyst." A number of different ways have been previously used to distribute liquid hydrazine into the catalyst bed, for breakdown of the incoming liquid hydrazine into the gaseous products, so that the hot gaseous products can product thrust.
One distribution method comprises the use of a manifold on the liquid hydrazine line with a plurality of small diameter tubes connected to the manifold to deliver the hydrazine to selected locations within the catalyst bed. The small diameter tubes are small bore hypodermic type tubes. Such structure has substantial volume of liquid in the manifold, which is necessarily in the vicinity of the hot catalyst bed because the small diameter tubes must be a fairly limited length. As a consequence of the tube and manifold volume, achievement of stable propellant flow after opening the valve takes a fairly long time. Furthermore, because of the heat soakback from the decomposition chamber through the tubes to the manifold subsequent to thruster firing, the temperature of the external manifold is raised above the auto-decomposition temperature of the hydrazine under some ON/OFF firing times. Under that circumstance, an attempt to fire the thruster by introduction of hydrazine into the manifold causes the hydrazine to detonate; this results in structural damage. Furthermore, with respect to this design, the small tubes are vulnerable to damage in handling and susceptible to fatigue due to vibration during launch. Such causes undesirable leakage of hydrazine.
Another prior art structure employs a porous mass of sintered material through which the hydrazine passes. The sintered material causes spreading of the liquid across the face of the catalyst bed. The hydrazine forms puddles on the sintered material, and, as a consequence of this, the hydrazine puddles periodically decompose to cause very large pressure surges and catalyst particle breakup. The fines generated by the catalyst particle breakup are expelled through the nozzle. The catalyst loss reduces useful life of the catalystic decomposition reactor. Furthermore, the sintered mass has considerable thermal mass and is directly adjacent the catalyst. This causes a rise in temperature after firing, with a result that there is auto-decomposition within the mass, if hydrazine is fed to the reactor before the sintered mass has a sufficient time to cool down.
Still another structure employs a perforated dome which discharges liquid streams into a void maintained by a screen. The other side of the screen carries the catalyst bed. In this construction, puddling again causes rough running and catalyst breakdown. Thus, these prior art constructions suffer from disadvantages.